JPH07294418A - System and method for detection of chemical substance as well semiconductor-laser-output buildup system - Google Patents

Abstract

PURPOSE: To provide a chemical substance detecting system, a chemical substance detecting method, and a semiconductor laser output build-up system capable of obtaining a stable output with a low loss. CONSTITUTION: This chemical substance detecting system is constituted of a semiconductor laser 119, an optical resonant cavity limited between the first and second inlet reflecting elements 120 and the second reflecting element 122, a return beam 136 coinciding with an incident beam 114 in the opposite direction, and a detecting system 130. A sample 128 is located in the interaction region of the cavity, and the detecting system 130 is arranged to detect the prescribed characteristic of the sample 128 in the interaction region. The detecting system 130 is located adjacently to the interaction region, and the laser 119 is optically locked with the cavity as a whole. The beam passes through the cavity practically with no loss except for the interaction with the sample 128.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chemical substance detection system, a chemical substance detection method, and a semiconductor laser output build-up system, and more specifically, to an optical system for detecting chemical substances such as chemicals, particularly a chemical substance. Self-locking optic cavity that generates an optical signal in response to the presence of an analyte.
al cavities).

[0002]

BACKGROUND OF THE INVENTION The sensitivity and dynamic range of optical technology is well suited for use in optical sensing systems. Therefore, various sensing devices are currently in use that seek to take advantage of these characteristics.

The light source is an important element in any optical sensing system. In choosing a light source, the designer must generally balance the often conflicting requirements of light output level, high efficiency, low cost, miniaturization, and structural strength. One useful development, especially in this regard, is that due to advances in semiconductor technology, coherent light sources (such as argon ion or helium-neon lasers) can be delivered from a few meters (such as surface emitting quantum well laser diodes) It is downsizing to micron. For example, semiconductor diode lasers with output power ranges from milliwatts to watts are now commercially available. Examples of such devices are described in Parke, R .; , Etc., “2.
0W cw, diffraction-limited
operation of a nomolithi
cally integrated master os
cillator power amplifee
r, "IEEE Photon. Tech. Let
t. , 5, pp. 297-300, (1993).

Wall plug (wa of diode laser)
Although the efficiency of 11-plug) is high, a light source that directly produces a high wattage of light output often requires a higher wattage power source to cool it. Therefore, sensing applications must balance the need for high wattage light output with the need for a compact portable device that consumes only a small amount of power.

One solution to this problem is to use light trapped inside the optical cavity. An optical cavity or resonator consists of two or more mirror surfaces arranged to capture the incident light and bounce back and forth between the mirrors. In this way the light inside the cavity will be orders of magnitude stronger than the incident light. This general solution is well known and has been used in various fields. For example, nonlinear frequency conversion (for example, Yariv
A. , Introduction to Optic
al Electronics, 2nd Ed. , Ho
lt, Rinehart, and Winston
See New York, 1976, Chapter 8), and, more broadly, Demtroder, W .; , Laser
Spectroscopy, Springer-Ver
lag, Berlin, 1982, pp. 390-39
That is the spectroscopic method as described in 5.

Extending this solution to chemical sensing involves the interaction of intracavity light with chemical analytes that emit light signals. The optical signal may typically be coherent or incoherent, and need not be at the same frequency as the intracavity light. The magnitude of the light signal depends on the amount of chemical analyte and the intensity of the light in the cavity. This technique has applications in gas monitoring, where the optical signal is generated by spontaneous Raman scattering. For example,
U.S. Pat. No. 4,648,714, "Molecular
r Gas Analysis by Raman S
cattering in Intracavity
Laser Configuration, ”Benn
er, et al., March 10, 1987.

In all of the above methods, the optical gain medium (such as a helium neon emitter tube) is inside the optical cavity. For a typical diode laser, the cavity mirror is placed directly on the diode gain medium itself. However, in some applications, such as frequency tuning and linewidth limiting, an antireflective coating is formed on one or both sides of the diode surface, which diode is defined by a mirror external to the diode. Operates inside the cavity. While diode gain media can operate inside such cavities, the low damage threshold of the diode emitting surface can result in build-up (boosting) of the output.
ld-up) amount is strictly limited. In other words, if the diode is placed inside the cavity, the output cannot be increased so much that the diode is damaged, and the maximum allowable output is often too small for an efficient and simple sensing scheme. Not suitable.

To overcome this limitation while producing a large optical field, the diode laser may be placed outside of another highly miniaturized optical cavity in which the diode laser's radiation is trapped. This alternative cavity will hereinafter be referred to as the "build-up" cavity. However, diode lasers emit coherent radiation with an optical bandwidth that is even greater than that of highly miniaturized buildup cavities. To achieve a substantial amplification of the diode laser radiation in the build-up cavity, the diode laser must be forced to emit coherent radiation with a linewidth close to or matching that of the cavity.

There are several well-known techniques for reducing the bandwidth of diode lasers. For example, active, all-electronic frequency locking of diode lasers is used. However, this technique requires a high speed servo with a bandwidth of 20 MHz or more, or even greater, and a large optical isolation of the diode laser from the cavity. Passive locking is
It has important advantages over active, all-electronic frequency locking. For example, the required electronic control would be greatly reduced, especially when narrow band radiation is needed, and no optical isolator would be required.

Alternatively, the substantial linewidth reduction may be implemented by an optical feedback scheme. For example, Dahman
i, etc. are "Frequency stability"
ion of semiconductor case
rs by resonant optical fe
edback, "Opt. Lett. , 12, pp. 8
76-878 (1987) reported passive optical locking of a diode laser into a build-up cavity. In this technique, the light from a diode laser is directed into the build-up cavity and if the frequency of the light matches the resonant frequency of the cavity, the light is trapped. At that time, some of the trapped light returns back towards the diode laser to act as a passive feedback mechanism that locks the frequency of the miniaturized diode laser to that of the miniaturized build-up cavity.

[0011] Tanner et al., "Atomic be
am collaboration using a la
ser diode with a self-loc
king power buildup cavit
y, "Opt. Lett. , 13, pp. 357-35
9 (1988) describe self-locking, output build-up cavities, which generate more than 1000 times more light in the cavity than the incident light, but they have this strong power only for optical excitation of cesium atoms. Utilizes an intracavity optical field. However, this technology
onsen, H .; R. But, "Frequency no
ise reduction of visible
InGaAlP laser diodes by d
differential optical feedback
methods, "IEEEJ. Quant. El
ec. , 29, pp. It has recently been applied to visible diode lasers, as reported in 877-884 (1993).

The disadvantage of the system mentioned by Dahmani et al., Tanner et al. Is to be done. However, a drawback of the weak optical locking technique is that it still requires careful electromagnetic control of both the magnitude and phase of the light fed back to the diode laser. This is described, for example, in US Pat. No. 4,907,237, “Optical Feedb.
ack Locking of Semiconductor
to Lasers, "Dahmani, B .; Etc., 1
March 6, 990; Hemmerich, A .; ,etc,
"Second-harmonic generati
on and optical stabilizat
ion of a diode laser in a
next external ring resonato
r, "Opt. Lett. , 15, pp 372-374
(1990); and Buch, P .; And Kohns,
P. , "Optically self-locked
semiconductorductor with
servo control for feedbac
k phase and laser curren
t, "IEEE. Quant. Elec. , 27, 18
63 (1991).

Self-locking, power build-up cavities have also been used for non-linear generation of particularly extreme amounts of coherent radiation. The use of low-to-moderate (<1%) feedback from the build-up cavity to optically lock a laser diode to the cavity is described, for example, in the article by Hemmerich et al.
And Dixon, G .; J. ． Tanner, C .; E.
And Wieman, C .; E. , "432-nm source
ce based on efficientsec
nd-harmonic generation of
GaAlAsdiode-laser radiat
ion in a self-locking ext
internal responsivity, "Op
t. Lett. , 14, pp 731-733 (198
9), and US Pat. No. 4,884,276, "Opt.
iCal Feedback Control in
the Frequency Conversation
of Laser Diode Radio
n, "Dixon, et al., November 28, 1989.

This concept is further described in W. Lenth and W.
P. US Pat. No. 5,038,352 by Risk, where stability of locking is achieved by using an anti-reflection (AR) film coated diode laser and strong (10% -50%) feedback. Is taught to increase. In addition, W. J. Kozlo
vsky, along with Lenth and Risk, Proc
needs of the Compact Blu
e-Green Lasers Topical Me
"Resonator-enhance" at eting
d frequency doubling in a
next extended cavitydiode la
ser, ”New Orleans, Louisian
a, Optical Society of Amer
ica, Feb. 1993, p. In PD2-1,
How they used strong (3%) optical feedback in the AR film coated diode laser and how they added a dispersive element that reflected the light emitted from the build-up cavity back through the cavity to the diode laser. Reported. Frequency stability was added by the dispersive element.

Passive all-optical frequency locking techniques for diode lasers are simpler and much more stable than all-electronic or weak optical feedback locking. Because it obviates the need for fine electronic control of the diode laser or the optical field.
In order to ensure that the diode laser is stably locked into the build-up cavity, the dominant optical feedback to the diode gain medium is from the build-up cavity rather than from any other source, for example , From the diode laser emitting surface, back to the diode gain medium.

Typically, the diode emitting surface is anti-reflection coated and the entire system can be considered a regular semiconductor laser with an intracavity etalon. The more light that is fed back from the build-up cavity to the diode laser, the higher the reflectivity of the diode emitting surface that allows stable optical locking.

Of all the above methods, only two examples employing all-optical locking with large feedback are Lenth and Risk (US Pat. No. 5,0,5).
38,352) and by Kozlovsky et al. However, none of these methods apply well to sensing applications. For the stated purpose of these systems, it is essential to halve the wavelength of the incident light, especially to convert the near infrared light to blue.

Siegman in Lasers, Univ
ersity Science Books, Mill
Valley, California, 1986, p.
p. As noted at 428-431, the power that can occur inside an optical buildup cavity is inversely proportional to the optical loss of the cavity. Optical loss is approximately the sum of all optical losses of individual intracavity elements such as mirrors.

A structure that is optimal for producing coherent radiation always leads to an effective amount of output radiation (Kozlovs).
See ky and Lenth). This conversion also acts as an additional loss mechanism for intracavity radiation,
It reduces the amount of power that can be generated in the optical cavity. Le
Nonlinear crystals used in systems such as those of Nth and Kozlovsky have additional absorption losses that act to reduce the fineness of the cavity. Both of these substantial loss mechanisms are essential for coherent radiation generation. However, intracavity chemical sensing, especially Raman gas sensing, requires as much intracavity power as possible, and thus any structure that incorporates unnecessary and real loss is sensitive to intracavity chemical sensing. Is not suitable for. The optimal structure for chemical detection is
It would be a build-up cavity with low optical losses that could be implemented, resulting in as little light as possible leaking out of the cavity, and thus Lenth
And those described by Kozlovsky.

It is well known in the context of external cavity semiconductor lasers that an anti-reflective coating on the diode output surface and strong optical feedback are essential to ensure stable performance. Rong-Qing, H .;
And Shang-Ping, T .; , "Improved
rate equations for extern
al cavity semiconductor l
asers, "IEEEJ. Quant. Elec. ，
25, pp. 1580-1584, 1989. These laser systems are used in intracavity spectroscopy, Baev, V .; M. Eschner, J .; , P
aeth, E .; Shuler, R .; , And Tosch
ek, P.E. E. , "Intra-cavity spe
ctrocopy with diode laser
rs, "Appl. Phys. B. , B55, pp. Four
63-477, 1992, but as noted above,
Intracavity miniaturization, eg, intracavity power, is carefully kept low to prevent optical damage to the laser surface of the diode. Therefore, these devices are unsuitable for sensing applications based on high intracavity power.

For chemical sensing, what is needed is a combination of high intracavity power and the required low power, passive and optical locking, thereby eliminating the need for complex and expensive locking circuitry. It is a compact system. The present invention provides a chemical substance detection system, a chemical substance detection method, and a semiconductor laser output buildup system that can obtain a high output with a low input (that is, a low loss) and a stable output. The purpose is to do.

[0022]

SUMMARY OF THE INVENTION A semiconductor laser output build-up system includes a semiconductor laser that has an emitting surface and outputs an incident beam. An optical resonant cavity is defined between the first entrance reflective element and one or more second reflective elements and has an intracavity beam along the intracavity optical path. The returning beam passes through the entrance reflective element,
It consists of a portion of the intracavity beam that coincides with the incident beam, but in the opposite direction. The laser is completely optically locked in the cavity and the intracavity beam passes through the cavity virtually lossless.

Preferably, the invention further comprises a wavelength determining element such as a diffraction grating (grating) or an etalon in the incident beam path between the laser and the first reflecting element.

The emitting surface preferably has a reflectivity of less than 0.01 or even less than 0.0001, for example due to antireflection coatings. The feedback to the laser through the emitting surface of the return beam may be strong, eg greater than 3%.

Also, preferably, the present invention may include an optical forming device in the incident optical path for impedance matching.

A preferred use of the system is in the detection of chemicals in samples. To that end, the sample is arranged in the interaction region of the cavity, and the detection system is arranged so that a predetermined characteristic of the sample (for example Raman scattered light) can be detected in the interaction region. The intracavity beam passes through the cavity substantially lossless except for interacting with the sample.

Another aspect of the invention is that the semiconductor laser is a high power diode laser having an unmodified emitting surface. In this case, the present invention provides a method of driving a diode laser with a current that is less than the threshold current of the bare diode.

[0028]

The present invention will be described primarily with respect to its use in gas sensing, which is particularly well suited for this application. However, the present invention may also be used for liquid sensing. The necessary changes are described below

A first and preferred embodiment of the invention is illustrated in FIG. The diode laser forms a light source 110, which has a emitting surface 112 and a back surface 113. The light source emits a beam 114. Examples of commercially available devices suitable as the light source 110 are Toshiba 9215, Hitachi HL6714.
There are high power, wide area devices such as the G or Philips CQL601D single longitudinal mode index guided (SLM-ID) laser as well as the Philips CQL602D.

In addition, the rear surface 113 of the light source 110 is
It may have low reflectivity (eg, as a result of antireflective coating). In this case, a small mirror should be placed behind the back surface of the light source to reflect the light emitted from the light source and return it to the light source through the back surface and along the line of beam 114. The advantage of this arrangement is that the position of this additional mirror can then be changed to adjust the system and also between the rear surface 113 and this additional mirror (described below).
It is also possible to place optical elements. This embodiment is shown in FIG.
Will be described in more detail below.

The circuitry necessary to enhance the light source 110 is known in the art and is not shown here. However, some suitable semiconductor laser devices, including those listed above,
Note that typically little or no additional power used to cool the device will result in a power output greater than about 100 mW. Further, other semiconductor devices such as antireflection coated diode lasers or superluminescent diodes may be used as the light source 110.

The light source 110 emits a beam 114, which is spatially formed in a known manner by a conventional optical forming system 116 to ensure proper impedance matching. The wavelength determining element 118 is included in the optical path of the beam 114 to set the value of the operating wavelength and limit the average optical bandwidth of the diode cavity. An additional conventional light beam forming system 135 would be needed for both wavelength determining element 118 and impedance matching to the build-up cavity if the beam 114 exiting the forming system 116 were not spatially optimized. The light source 11 is manufactured by using a known manufacturing technique.
0, the optical shaping system 116 (and the light beam shaping system 135, if any), and the wavelength determining element 118 may be mechanically incorporated into a single light source unit, as indicated by dashed line 119. The optical shaping system 116 and the wavelength determining element 118, which act as an optical tunable bandpass filter, may be conventional devices whose construction and operation are well known. Beam 114
Forms the incident beam, which is indicated by the right-pointing arrow in FIG.

Beam 114 impinges on a build-up cavity consisting of reflective elements such as mirrors 120, 122 and 124 to produce intracavity beam 126. The mirror 120 is preferably tilted (but not perpendicular to the direction of the beam 114) to prevent direct reflection of the beam 114 back to the light source. This would reduce the ability of the light source to lock the intracavity beam 126. Beam 11 reflected by tilted mirror 120
4 and a very small portion of the beam 126 through mirror 120 (from mirror 124) is indicated by dashed arrow 125.

Beam 126 passes through sample 128 (contained in any conventional cell or other holding structure),
The optical signal generated by the interaction between the sample 128 and the beam 126 is detected by the conventional detection system 130. For example, Raman scattered light may be detected. , The output from the detection system is
It becomes an input signal to the conventional processing circuit 132, and the presence or amount of the target substance in the sample 128 is determined by using a known analysis technique. The analysis results by the processor may be further passed to processing circuitry (not shown) or may be displayed on a conventional display device 134.

A small portion of beam 126 leaks as return beam 136 through entrance mirror 120. Return beam 136 is opposite in direction (as indicated by the left-pointing arrow) but coincides with incident beam 114. Entrance mirror 120
The return beam 136 from the wavelength determining element 118, the optical forming system 116 (and the light beam forming system 135, if any)
Through the emitting surface 112 and the light source 110
Return to.

The present invention is a conventional stable current source (not shown).
Otherwise, the output of the intracavity beam 126 is orders of magnitude greater than the incident beam 114 without any electronic control of the light source. Yet another advantage is
It is to generate an extremely large optical signal from a sample by a small and inexpensive light source. This includes emitting surface 112, optical forming system 116 (and light beam forming system 135, if any),
Wavelength determining element 118 and mirrors 120, 122, 12
4 parameters to maximize the power of the intracavity beam 126 as described below.

To ensure maximum intensity of the intracavity beam 126, most of the output from the light source 110 must be resonant (for the same frequency) with the buildup cavity. In other words, the light source should be optically locked in the cavity. According to the present invention, this is accomplished by ensuring that the dominant feedback to the light source 110 is the return beam 136. The main problem in this regard is the reflection of the beam 114 back to the light source 110 by the emitting surface 112.

Return beam (leakage beam) according to the invention
Any of several methods may be used to ensure that 136 is the dominant feedback. If the diode laser (light source) 110 is Toshiba 9215, Hitachi 6
714G or Philips CQL601D type SL
If it is an M-ID device, it usually works well above the threshold current of the bare diode (current threshold for an unmodified diode laser operating separately) for optimal output buildup. Calculations and experiments have shown that the return beam 136 has feedback in the range of 1% to 50% to ensure power stability. To ensure that the beam 136 is the dominant feedback, the reflection of the beam 114 from the emitting surface 112 must be at least 10 to 100 times less intense than the beam 136. in this case,
The emitting surface 112 should have a reflectivity of approximately 0.01 or 0.001 and it should preferably be on the order of between 0.001 and 0.0001 to ensure stable locking. is there. In one prototype of the invention, the cavity feedback is 10% and 20%.
It is estimated that it is between and Hitachi 67 for stable rocking
The reflectance of the emitting surface of the 14G type diode laser is 0.0
It was within the range of 01 to 0.0001.

One way to obtain these low reflectances is by using an antireflection coating on the surface. This reflectance reduction technique is known (eg, E
isenstein, G .; And Stulz, L .; W. ，
"High qualityantireflecti
on coatings on laser face
sts by sputtered silicon
nitride, "Apple. Opt. , 23, 16
1 (1984)). According to the present invention, one advantage of anti-reflective coatings applied to diode lasers is that they do not require the separate electronic circuitry conventionally required to phase lock the diode in the cavity.

Alternatively, high output S without changing the emitting surface
LM-ID or other diode lasers may be used. If the feedback is large enough, lasing of the diode-cavity system will occur below the threshold current of the bare diode. As long as the diode-cavity system produces a laser, the reflection of light from the emitting surface must be low enough not to deplete the diode gain. As the diode current increases near the bare diode threshold, radiation reflections will begin to reduce the diode gain, which in turn will result in output instability.

Therefore, the limit of the output buildup is determined by the feedback amount and the radiation surface reflectance. One advantage of the present invention is that it can operate without any of the aforementioned feedback limiting devices such as variable attenuators or optical isolators. For example, a high power diode would work well below the threshold current of the bare diode and still have sufficient gain to generate a laser when light is fed back from the buildup cavity.
Such diodes are described in Ueno, et al., “30-mW
690-nm high-power strain
d-quantum-weee AlGaInP la
ser, "IEEE J. Quant. Elec. , 2
9, 1851 (1993); and Arimoto, et al.,
"150mW Fundamental-transv
erse-mode operation of 67
0nm window laser diode, IE
EE J. Quant. Elec. , 29, 1874
(1993). One prototype of the present invention used a 680 nm SLM-ID laser with a back surface of a high reflector and an emitting surface with 10% reflectivity. This diode is below the threshold current of the bare diode 10
Since it operates at mA and the effective reflectance of the build-up cavity exceeds 10%, an appropriate stable output build-up was achieved.

Other sources of reflection back to the diode laser should be minimized. For example, the optical forming system 116 (and the light beam forming system 135, if any) and the wavelength determining element 118.
Should be anti-reflective coated or simply slightly tilted.

In order to maximize the intensity of beam 126, optical forming system 116 (and light beam forming system 13 if present) is used.
5) includes elements such as lenses or prisms that form the beam 114 and spatially align the beam 126. In addition, the wavelength determining element 118 will operate optimally due to the unique characteristics of the incident beam. For example, if the wavelength determining element 118 is an etalon, then the optimal incident beam must be non-divergent to pass the maximum amount of light. In this case, the optical element 116 optimally forms the beam 114 so that it can pass through the element 118, and the optical element (light beam forming system) 135 spatially aligns this passing light with the beam 126. Note that the wavelength determining element 118 can also form the beam 114, if it is a diffraction grating, for example. In some applications, sub-optimal performance of element 118 may be acceptable, in which case the beamforming function of element 135 may be incorporated into element 116.

Both the components and techniques for beamforming described above are well known in the optical arts and conventional components may be used. The shape of beam 126 is mirror 12
There may be one or more transverse electromagnetic cavity modes, determined by the build-up cavity formed by 0, 122, and 124.

There are many options in the design of optical shaping system 116 (and light beam shaping system 135, if any) known to those skilled in the art. A suitable configuration example is as follows. (A) A single lens or multiple lenses for collecting the beam 114 and focusing on the build-up cavity. (B) A single lens or multiple lenses for collimating the beam 114, and an additional lens for focusing the beam 114 into the build-up cavity. In this case, the position and focal length of the additional lens is selected to impedance match the beam 114 to the buildup cavity. (C) Two cylindrical lenses for circulating the beam 114 in addition to (b). (D) Same as (b), plus a pair of cylindrical prisms for circulating the beam 114. (E) Same as (c) or (d), plus an astigmatism correction lens. (F) A GRIN lens for collimating the beam 114 and a second lens for focusing the beam on the build-up cavity.

The configurations (a)-(f) are all beam 114
To the build-up cavity with different efficiencies. To compensate for this effect, it would be necessary to modify the diode current to produce the same light output. The imaging system with the highest power must be chosen so that the beams 126 and 136 are most intense at a given diode laser current. This choice depends on the particular geometry for a given application, as well as the cost, and may be made using known empirical theoretical techniques.

The build-up cavity is D = c / 2L
It has a "comb-shaped" resonant frequency with a spacing of. Where D is the mode interval in Hertz, c is the speed of light, and L is the round-trip length of the build-up cavity.
path length). For example, if L = 3
At 0 cm, D = 0.50 GHz. If the intracavity beam 126 should contain one or several cavity modes for a particular sensing application, then a wavelength (or frequency determinant) 118 should be employed.

The gain medium of a typical diode laser is
It has a substantial gain over a width of about 3 THz (ie 5 nm at 670 nm), so that many cavity frequencies generate lasers in random states simultaneously or separately,
And you will be able to build up. Wavelength determining element 11
The purpose of 8 is to limit the number of build-up cavity modes that can generate a laser. The number of lasing modes depends on the band of the wavelength determining element 118, and may be one. To ensure that the beams 126 and 136 are as large as possible, the transmittance of the wavelength determining element 118 is
Must be big. In various working prototypes of the present invention, a transmittance of 80% or greater was used.

Wavelength determining element 118 includes a suitable filter element, which may be dispersive, absorptive, or a combination of both. Optical filters or absorbers with narrow bands may be used to heavily attenuate the majority of modes and thus prevent them from multiplying. Examples of suitable filters include dielectric stack bandpass filters, etalons, Lyot filters, and ultrasonic optical filters. The element 118 may be incorporated into the back surface 113 of the light source 110, for example by depositing a dielectric stack reflector or other known coating on the back surface 113. In this case, the effect of the back surface 113 is to reflect only a predetermined, appropriately narrow frequency band back along the optical path of the beam 114, thereby limiting the number of cavity modes in which the laser can be generated.

Dispersive elements, such as diffraction gratings and prisms, polarize the beam by an angle (depending on the frequency of the light). Therefore, only a few build-up cavity modes can occur in beam 126 and provide beam 136 back to diode laser source 110. in this case,
The number of modes that can be built up is determined by the dispersion of the wavelength determining element 118 and the distance between this element and the buildup cavity.

An example of a suitable diffractive element is a transmission grating with a transmission efficiency of 80% and 1800 grooves per mm. A reflection grating may be used. In the prototype of the present invention, mm
A grating having 2400 grooves per side and a reflection efficiency of 90% was used. Beam 114 may be polarized (or reflected) as it passes wavelength determining element 118.

FIG. 2 illustrates a second embodiment of the present invention, in which the rear surface 113 is AR film coated, as is the emission surface 112, and an additional mirror 200 is provided.
Reflects the beam 114 back to the light source 110 through the back surface 113. The wavelength determining element 118, if it is now placed between the mirror 200 and the back surface 113, would operate in the same manner as described above. In addition, optical shaping system 116 may be required to form beam 114 for optimum performance of element 118. In this case, this too would be located between the mirror 200 and the back surface 113, and preferably (but not necessarily) between the wavelength determining element 118 and the back surface 113.

If the frequency window (ie, bandpass filtering) passed by wavelength determining element 118 could be tuned, then the average frequency of beam 126 could also be tuned. The tuning range is ultimately limited by the gain band of the diode laser source 110. Examples of tuning include rotating either the grating or the etalon. In the present invention, the wavelength determining element 118 is placed between the laser light source 110 and the cavity. The element 118 is now the rear surface 113 of the light source.
Light source and additional external mirror 2
You can put it between 00 and. The location of element 118 in the present invention is therefore radically different from that described by Kozlovsky, for example. The cavity design of the present invention allows a negligible amount of light to pass through the mirrors 122 and 124.

Mirrors 120, 122, and 124 each have a radius of curvature and relative spacing selected to meet the needs of any particular sensing application, using conventional criteria in combination to optimize the intensity of beam 126. There is. The reflectivity value of the mirror determines the fineness of reflectivity of the optical cavity and thus the intensity of the beams 126 and 136. The actual value will depend on the given application, and it may be determined empirically or by calculation using known techniques whose main consideration is impedance matching. According to calculations, the optimal impedance matched mirror 120 is
It will have losses (including transmission, absorption, and scattering losses) in the range of 50-300 (ppm), and the losses of the second mirrors 122, 124 will be in the range of 1-100 ppm. It is shown. Two examples of transmission loss of suitable mirrors used in the prototype of the present invention are when all three mirrors have a transmission loss of 12 ppm, or when the mirror 120 has a loss of about 100 ppm and the mirror 122 And 124 have losses of about 12 ppm. Comparing a system using a diffraction grating to another frequency locking device behind the cavity (away from the cavity's "end mirror") in the optical path,
The end mirrors 122,124 of the present invention may therefore reflect several orders of magnitude, significantly reducing losses and increasing the efficiency of the present invention.

The optimum radii of curvature of the mirrors 120, 122, and 124 also depend on the given application and can be determined experimentally or theoretically. In one prototype of the present invention, the radius of curvature of all mirrors 120, 122, and 124 was 17 cm. The distance between the mirrors 120 and 122 and the distance between the mirrors 120 and 124 should also be selected in any conventional manner to form a stable cavity mode. In the prototype of the invention just described, the spacing between the mirrors was 14 cm.

If the sample 128 contains a large amount of gas which fills the beam path of the cavity, the mirrors 120, 1
The angle between the two arms of the V-shaped buildup cavity formed by 22 and 124 should be small (eg, less than 20 degrees). Otherwise, the change in refractive index due to the change in gas will affect the refraction of the beam 114 as it passes through the entrance mirror 120. Other mirror positions and radii of curvature may be suitable for other applications, such as bringing the mirrors 124 and 120 closer together to fit within one small solid mount.

The detection system 130 incorporates known optics suitable for collecting the generated signal and a detector for detecting the collected signal. The position of the detection system 130 is
Depending on the location of the optics employed in the detection system 130, it may be next to the sample 128 or some other suitable location.
The placement of the detection system is usually application dependent and can be readily determined by experimentation or knowledge of the properties of the optical signal (such as Raman scattering or fluorescence) that the sample is expected to emit.

The three-mirror, V-shaped build-up cavity characterized by mirrors 120, 122, and 124 is not the only preferred cavity configuration. Other configurations are described below. However, a general property of the inventive build-up cavity is that it ensures a large amount of optical feedback to the diode laser. Typically,
A cavity configuration should be chosen that results in a single return beam 136 originating from the intracavity beam 126.
The V-shaped cavity accomplishes this because any non-resonant incident beam will be reflected at the entrance mirror 120 and not back to the diode laser. Another way to achieve this would be by adding extra cavity mirrors, as needed for the ring cavity. A preferred example without the need for additional elements is shown in FIGS.

FIG. 3 illustrates a third embodiment of the present invention, in which an optical cube 310 replaces the mirrors 120, 124 shown in FIG. A feature common to both embodiments of FIGS. 2 and 3 is that the reference numbers in FIG. 1 are used. In particular, it is not necessary to integrate it into a single unit as in the first embodiment, but an integrated light source,
The forming optics and the wavelength determining element are numbered 119.

Optical cube 310 is made by optically cementing two prisms 312 and 314 together in a known manner. In one trial, two right angle prisms were used. One surface of prism 312 (effectively parallel to incident beam 114 and return beam 136) reflects strongly, while surfaces 316 and 318
(In effect, the entrance and exit surfaces of optical cube 310, respectively, perpendicular to the beam) are anti-reflection coated. In order to produce an intracavity beam 322 with as much intensity as possible, the surface or face 318 should be as low loss as possible. Third surface 324 of prism 312 (the surface at the contact surface of the two prisms)
Is also strongly reflected and its reflectivity value is chosen to impedance match the input beam 114 to the cavity in any known manner.

The optical cavity comprises three mirrors 320,
324 and 122. The advantage of this configuration is
That is, two optical elements are integrated into one. However, care must be taken to reduce the birefringence of the prism material. Polarization-rotation element (polarizati)
An on-rotation element 326, preferably a half-wave plate, is required when the beam 114 is polarized and could be adjusted using known prior art techniques.

FIG. 4 shows again a suitable light source 1 according to the invention.
4 illustrates a fourth example of an optical cavity having 19 In this embodiment, the cavity configuration is linear, having an entrance mirror 410 that impedance-matches beam 114 to the cavity by its reflectivity and is formed by mirrors 410 and 122.

In this case, there is a reflection from mirror 410 exactly back along beam 136, which consists of two components. The first reflected component consists of light at the resonant frequency of the cavity and the second component consists of light at all other incident frequencies. The first resonant component is the mirrors 410 and 12
2 parameters (curvature and reflectivity) and beams 114 and 1
It is measured by both the degree of spatial overlap between the 26. The magnitude of the first reflected component may be considered as the amount of light in the cavity that leaks back through the mirror 410, whereas the second component is determined by the parameters of the mirror 410 alone (curvature and reflectivity). Both of these reflected components provide feedback to the light source. The magnitude of this feedback is further determined by the beam forming optics.

If it is similar in magnitude to the first resonant feedback path, the second feedback path will compete with the optical gain at the light source, leading to output instability. In order to stably confine the light source to the first resonant feedback reflection, it must be guaranteed that the magnitude of the second feedback is about 10 times smaller than the first resonant feedback. in this case,
For example, the drive current for the light source may be selected to provide only enough gain so that the system is above the lasing threshold for the first resonant feedback only. The limitation of this solution is the inability to further increase the gain of the light source due to output instability. Therefore, the amount of power that can be built up in the cavity formed by mirrors 410 and 122 will be limited.
Another and preferred solution is to include a wavelength determining element (included in unit 119 in FIG. 4) and place it between the light source and mirror 410.

The frequency spacing between the resonant modes of the optical cavity is inversely proportional to the mirror separation distance. If the light source (or more precisely, the high reflector of the light source, ie the rear face 1
If the distance between either mirror 13 or mirror 200) and mirror 410 is less than the distance between mirrors 410 and 122, the frequency difference between the resonant modes in the former is greater than in the latter. For example, if a narrow bandpass filter were to be used for bandpass filtering of a magnitude less than the just-mentioned frequency difference, the second feedback path would be It will turn off the laser. Similar solutions would be possible for the wavelength determining elements mentioned above, especially when they are provided as etalons.

Another way to separate the non-resonant reflections is by polarization. If a low-loss polarization-rotating element, such as a half-wave plate, is included in the optical cavity, the resonant reflection is out of phase with the non-resonant reflection and can be distinguished by inserting a polarizer between the light source and the cavity. If the light source emits substantially polarized radiation, then additional polarization-rotation elements would be required.

In the embodiments described for the present invention,
The beam in the cavity is reflected by the entrance reflection element (mirror 12
0 and 410 or prism pair 310), through the sample, to the end mirror 122, shown as going straight. This is the simplest and therefore, although not necessary, the preferred intracavity optical path when the present invention is used to understand physical properties such as Raman scattering by a sample. Instead, the intracavity beam could be directed to one or more intermediate reflective elements with the sample properly positioned with respect to the intracavity beam and detection system 130. However, the intermediate element is strongly reflected so that lossless buildup is maintained in the cavity,
And only if the cavity geometry (spacing and radius of curvature) and its limiting reflective elements are adjusted in any conventional manner to ensure that a stable cavity mode is formed.

Such an arrangement may be useful, for example, in applications where the intracavity beam does not pass directly through the sample, but rather the signature of the sample of interest is detected based on some other interaction between the beam and the sample. , May be necessary. In either case, the sample will be placed in an interaction region of the optical cavity, which may be inside the cavity itself (as shown in the figure). Moreover, the optical loss of the sample should be less than that of the cavity mirror. If not, the reflectivity of the mirror must be chosen to optimize impedance matching to the cavity.

In various embodiments of the invention, it is not necessary to intentionally include some loss mechanism in the cavity, such as a nonlinear crystal. One major advantage of all embodiments of the present invention is that they enable substantially lossless reflection over the entire spectrum of laser diode radiation. In fact, there is no inherent structural loss in the cavity, the only "loss" is absorption by the sample,
Only those that have been converted or scattered, of course, are related to the energy detected by the detection system 130.

The present invention is not limited to the above embodiments. That is, it has various preferred embodiments as described below.

That is, the chemical substance detection system of the present invention comprises: [1] A) a semiconductor laser having a radiation surface and outputting an incident beam along the incident beam path; and B) a first entrance reflecting element. An optically resonant cavity defined between the at least one second reflective element and having an intracavity beam along the intracavity beam path; and C) comprising a portion of the intracavity beam transmitted through the entrance reflective element, A chemical detection system in a sample, comprising: a return beam that is coincident with the incident beam but in the opposite direction; and D) a detection system, wherein E) the sample is placed in the interaction region of the cavity The detection system is arranged so that it can detect certain properties of the sample within its interaction area; F) the detection system is placed next to the interaction area; G) the laser is H) is characterized in that the beam inside the cavity passes through the inside of the cavity with virtually no loss except for the interaction with the sample. It has such a preferred embodiment.

[2] Further, a wavelength determining element is included in the incident beam path between the laser and the entrance reflecting element [1].
The system described in.

[3] The system according to [2], wherein the wavelength determining element is a diffraction grating.

[4] The system according to [2], wherein the wavelength determining element is an etalon.

[5] Entrance reflection element is 50 to 300 pp
m has a loss in the range of m, and each second reflective element is 1 to 1
The system according to [1], which has a loss in the range of 00 ppm.

[6] The system according to [1], wherein the feedback of the returning beam to the laser through the emitting surface is larger than 3%.

[7] The system according to [1], wherein the radiation surface is coated with an antireflection film.

[8] The system according to [7], wherein the emitting surface has a reflectance of less than 0.01.

[0079]

[9] The system according to [8], wherein the emitting surface has a reflectance of less than 0.0001.

[10] The system according to [1], further including an optical forming and impedance matching device in the incident beam path.

[11] The system according to [1], wherein the entrance reflection element is placed at an angle such that it is not perpendicular to the incident beam.

[12] Further, including a rearview mirror,
Here, A) the semiconductor laser has a rear surface, and B) the rearview mirror reflects the light exiting the rear surface and returns it to the semiconductor laser through the rear surface [1].
The system described in.

[13] The system according to [12], further including a wavelength determining element disposed between the rearview mirror and the rear surface.

[14] The system according to [13], wherein the rear surface is coated with an antireflection film.

[15] The system according to [14], which further includes optical forming and impedance matching means for optically forming and impedance matching the light emitted from the semiconductor laser.

[16] The system according to [1], wherein the semiconductor laser has a rear surface coated with a reflection film having a narrow bandwidth.

[17] The system according to [16], wherein the reflection film having a narrow bandwidth is a dielectric stack.

[18] The system according to [17], further including optical forming and impedance matching means for optically forming and impedance matching the light emitted from the semiconductor laser.

Further, the chemical substance detection method of the present invention is described in [1
9) A) directing the incident light from the semiconductor laser as an incident beam into a substantially lossless, internally unobstructed optical resonant cavity; B) globally locking the laser with respect to the optical resonant cavity; [20] to [22], characterized in that C) arranging the sample in the interaction region of the cavity, and D) detecting a predetermined characteristic of the sample in the interaction region.
The preferred embodiment is as follows.

[20] The method according to [19], further comprising the step of feeding back a returning beam having an intensity of at least 3% of the incident beam to the laser.

[21] The method according to [20], wherein the semiconductor laser is a diode laser having an unmodified emitting surface, and the step of driving the diode laser with a current smaller than the threshold current of the bare diode is included.

[22] The method according to [20], further including the step of coating the emitting surface of the semiconductor laser with an antireflection film.

Furthermore, the semiconductor laser output build-up system of the present invention is [23] A) a semiconductor laser which has an emitting surface and outputs an incident beam along an incident beam path, and B) a first entrance reflecting element. And an at least one second reflective element, the optical resonant cavity having an intracavity beam along the intracavity beam path, and C) comprising a portion of the intracavity beam transmitted through the entrance reflective element, A return beam that matches the beam but is in the opposite direction, D) the laser is optically locked into the cavity, E) the intracavity beam passes through the cavity substantially lossless. [24]-
It has a preferred embodiment such as [39].

[24] Further, a wavelength determining element is included in the incident beam path between the laser and the entrance reflecting element [2.
The system according to [3].

[25] The system according to [24], wherein the wavelength determining element is a diffraction grating.

[26] Entrance reflection element is 50 to 300 p
has a loss in the range of pm and the second reflective element is 1 to 1
The system according to [23], characterized in that it has multiple losses in the range of 00 ppm.

[27] The system according to [23], wherein the emitting surface has a reflectance of less than 0.01.

[28] The system according to [27], wherein the emitting surface has a reflectance of less than 0.0001.

[29] The system according to [23], wherein the feedback of the return beam returning to the laser through the emitting surface is more than 3%.

[30] The system according to [29], wherein the radiation surface is coated with an antireflection film.

[31] The system according to [23], further including an optical shaping and impedance matching device in the incident beam path.

[32] The system of [23], wherein the entrance reflective element is angled such that it is not perpendicular to the incident beam.

[33] Further, including a rear-view mirror, A) the semiconductor laser has a rear surface, and B) the rear-view mirror reflects the light emitted from the rear surface and returns it to the semiconductor laser through the rear surface. Features [23]
The system described in.

[34] The system according to [33], further including a wavelength determining element disposed between the rearview mirror and the rear surface.

[35] The system according to [34], wherein the rear surface is coated with an antireflection film.

[36] The system according to [35], further including optical forming and impedance matching means for optically forming and impedance matching the light emitted from the semiconductor laser.

[37] The system according to [23], wherein the semiconductor laser has a rear surface coated with a reflection film having a narrow bandwidth.

[38] The system according to [37], wherein the reflection film having a narrow bandwidth is a dielectric stack.

[39] The system according to [38], further including optical forming and impedance matching means for optically forming the light emitted from the semiconductor laser and impedance matching.

[0110]

As described above, according to the present invention, a chemical substance detection system and a chemical substance detection system capable of obtaining a high output with a low input (that is, a low loss) and a stable output can be obtained. A method and a semiconductor laser output buildup system can be provided. That is, according to the present invention, the output of the intracavity beam can be increased by orders of magnitude compared with the incident beam without requiring any electronic control of the light source. Further, since a separate electronic circuit system for phase-locking the cavity is not required, a very small optical source can generate an extremely large optical signal from the sample.

[Brief description of drawings]

FIG. 1 illustrates a first embodiment of the invention illustrating a sensing system having an optical locking semiconductor light source external to the output buildup cavity.

FIG. 2 is a diagram illustrating a second embodiment of the present invention in which an external mirror reflects light and returns it to a light source through a rear surface coated with an antireflection film.

FIG. 3 is a diagram illustrating a third embodiment in which an optical cube is replaced with a plurality of mirrors in the embodiment shown in FIG.

FIG. 4 is a fourth aspect of the invention in which the cavity has a linear configuration.
It is a figure explaining the Example of this.

[Explanation of symbols]

110 Light Source (Laser) 112 Radiation Surface 113 Rear Surface 114 Beam 116 Optical Forming System 118 Wavelength Determining Element 119 Unit of Light Source 120 Entrance Mirror 122,124 End Mirror 125 Part of Beam 126 Intracavity Beam 128 Sample 130 Detection System 132 Processing Circuit 134 Display Device 135 Light Beam Forming System 136 Return Beam 200 Additional Mirror 310 Optical Cube (Prism Pair) 312, 314 Prism 316, 318 Entrance and Exit Surfaces of Optical Cube 310 322 Intracavity Beam 324 Third Surface of Prism 312 326 Polarization-rotation element 410 Entrance mirror

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Claims (3)

[Claims] 1. A) a semiconductor laser having an emitting surface and outputting an incident beam along an incident beam path; and B) between a first entrance reflecting element and at least one second reflecting element. Consists of an optical resonant cavity with a limited, intracavity beam along the intracavity beam path, and C) a portion of the intracavity beam that is transmitted through the entrance reflective element, coincident with, but opposite from, the incident beam. A return beam and D) a detection system, E) the sample is placed in the interaction region of the cavity, the detection system being arranged to detect a predetermined property of the sample within the interaction region, F) the detection system is placed next to the interaction region, G) the laser is optically locked into the cavity, H) the intracavity beam interacts with the sample Chemical sensor system in a sample outside the pass through the cavity inside substantially lossless, and said. 2. A) directing incident light from a semiconductor laser as an incident beam into a substantially lossless, internally resonant optical resonant cavity; B) globally locking the laser with respect to the optical resonant cavity. C) arranging the sample in the interaction region of the cavity, and D) detecting a predetermined characteristic of the sample in the interaction region, the method for detecting a chemical substance in a sample. 3. A) a semiconductor laser having an emitting surface for outputting an incident beam along an incident beam path; and B) defined between a first entrance reflecting element and at least one second reflecting element, An optically resonant cavity having an intracavity beam along the intracavity beam path, and C) a return beam consisting of a portion of the intracavity beam transmitted through the entrance reflecting element, which is coincident with the incident beam but opposite thereto. A semiconductor laser output build, characterized in that D) the laser is totally optically locked to the cavity, and E) the intracavity beam passes through the cavity substantially lossless. Up system.